A flower boy is a boy who both looks and acts like a girl. They tend to have feminine habits, such as wearing makeup, and will also be much prettier than any girl you will ever see.
Girl 1: Have you ever seen Ren from Nu'est?
Girl 2: He is such a flower boy.
Girl 1: I know right! Why can't I be as pretty as he is?
Flower Boy in the LGBTQ community can be used to refer to any male/masculine identified individual (i.e. cis gay male, trans man) who also has a feminine side, and is soft and feminine in either personality, self-expression/style, or both. The soft boi variation of the word can also refer to someone who considers themself only partially male/masculine.
A Flower boy: growing up I was always told I was "too soft" and needed to "man up", I'm glad that I'm just a proud flower boy who isn't ashamed of that anymore."